A gas turbine engine combustor is typically disposed within an annular combustion section between an inner and an outer engine case wall. A plurality of primary fuel nozzles disposed in the upstream end of the combustor supply a mixture of fuel and air axially into the combustor at a closely controlled ratio. A plurality of secondary fuel nozzles are disposed in the outer engine case wall. The secondary fuel nozzles supply a mixture of fuel and air radially into the combustor during engine startup and at certain thrust levels. The secondary fuel nozzles are actuated during low and intermediate power regimes to stabilize the flame in the combustor.
Typically, the secondary fuel nozzles include a central axis about which are disposed an inner and an outer concentric fuel tubes. The inner tube carries liquid fuel while the outer tube carries fuel supplied as a gaseous fluid (natural gas fuel). The gaseous fuel in the outer tube thermally insulates the liquid fuel in the inner tube thereby preventing a problem of coking within the fuel nozzle. Coking is a thickening of any residual fuel that is stagnant within the fuel system passages. When stagnant fuel is heated, it solidifies and can reduce effective fuel flow capacity and actually plug the fuel supply system. The secondary fuel nozzles are particularly susceptible to coking because fuel tends to stagnate and get heated within the nozzle when the nozzle is not actuated during those thrust settings when only the primary nozzles are operating. Thus, insulating the inner tube carrying liquid fuel by the outer concentric tube, reduces the problem of coking.
However, the geometry of the inner and outer concentric tubes is not without problem. It will be appreciated that the environment within a gas turbine engine combustion chamber is extremely harsh. The fuel-air mixture burns in the combustion chamber at temperatures as high as 2100.degree. C. (3800.degree. F.) causing extreme thermal gradients and therefore, thermal stresses in the inner and outer engine case walls in the combustion section. Moreover, rotational movement of the engine's compressor and turbine, as well as the high flow rate of the fuel-air mixture and the burning thereof, may cause significant vibration and pressure pulsations in the combustion section and engine case walls. Such high thermal stresses and vibration experienced by the combustion section walls are also experienced by the secondary fuel nozzles. Prior art secondary fuel nozzles have, in large measure, failed to adequately tolerate such a harsh vibratory and thermal environment without themselves exhibiting vibratory movement. Such movement risks not only the misalignment of the fuel nozzles with other components in the combustor such as igniters, and the like, but also actual damage to the concentric fuel tubes of the nozzles due to relative vibratory movement between the inner and outer fuel tubes. The inner and outer fuel tubes may crack due to wear and fatigue caused by the vibratory stresses.
U.S. Pat. Nos. 3,785,407 to Waite et al. and 4,098,476 to Jutte et al. teach an apparatus for a spacer member between a pipe and a cover, and a support apparatus to prevent rotational and translational motion at certain temperatures respectively. While Waite et al. discloses a pipe cover spacer with yieldable fingers extending to make contact with a pipe, it is desirable to dampen vibrations between two tubes in an economical way. The yieldable fingers in Waite's disclosure are separate pieces arranged circumferentially to provide a spacing function. Further, while Jutte et al. discloses a support apparatus that fits loosely around the inner housing, this configuration would not be able to dampen low amplitude vibrations between two concentric tubes. In addition, the support apparatus is Jutte et al. is a circumferentially continuous ring, a configuration which would impede flow in the annulus of the outer tube. Thus, there is a need to provide an economical vibration damping system for two concentric tubes, while maintaining fuel flow in the outer tube.